This invention relates to data communications techniques used in modems. More particularly, this invention relates to a method and apparatus for an originating modem to identify an answering modem.
In a dial-up communications network, e.g., the public switched telephone network (PSTN), an originating modem does not necessarily know what type of modem will answer the data call. As a result, the originating modem typically assumes that all data calls are made to a "generic" modem and that all "initial handshaking" and initial communications must be done according to established industry standards like International Telegraph and Telephone Consultative Committee (CCITT) V.25. For example, in order to establish a data connection, the originating modem dials the telephone number of the location where the answering modem is located. The answering modem detects the ringing signals on the telephone line and answers the telephone call. At this point the answering modem places an "answer tone" on the telephone line. The originating modem detects the answer tone and then places an "originating carrier" on the line. When the answering modem detects the originating carrier, the answering modem places an "answering carrier" on the line and the initial handshaking process is completed, i.e., a physical data connection is established between the originating modem and the answering modem. From this point forward, the originating modem and the answering modem can execute additional handshaking and protocol procedures (like CCITT V.32) to further establish the desired level of connection, e.g. the desired baud rate, communications protocol, etc.
As described above, a data communications industry standard, like CCITT V.25, provides a common reference point whereby a modem manufacturer can ensure its modem will be able to communicate with modems produced by other manufacturers. However, in offering data communications services to a customer, the data communications industry standards may not provide all of the features that a customer may desire, or may not provide the features in a manner that suits a particular modem manufacturer. For example, a modem manufacturer may be of the view that a network management feature properly requires that a secondary communications channel also be established between the originating modem and the answering modem--however, available industry standards may not provide for this type of a secondary channel. Another example is to allow faster connection between the modems by shortening the time required to determine modem types.
As a result, after the establishment of the switched data connection between an originating modem and an answering modem, it may be desired to switch to a proprietary form of operation in order to offer non-industry standard features to a customer. Consequently, this requires that the originating modem and the answering modem be the same type of modem and requires these modems to perform some additional handshaking process in order to identify each other.
Various techniques have been disclosed in a number of U.S. Patents which provide for an originating modem and an answering modem to identify each other. U.S. Pat. No. 4,215,243, issued to Maxwell on Jul. 29, 1980, provides an ability to generally identify a modem as to the type of industry standard protocol it supports (as opposed to identifying a particular manufacturer's modem) by identifying the frequency of the originating carrier after the answer tone has been provided from the answering modem. U.S. Pat. No. 4,680,773, issued to Amundson on Jul. 14, 1987, discloses a technique for sending special characters after the physical data connection is established. These special characters, when detected, allow the originating modem and answering modems to identify each other for changing to a proprietary form of operation. Finally, U.S. Pat. No. 4,782,498, issued to Copeland, III on Nov. 1, 1988, establishes a special mode for proprietary use by means of a special handshake procedure. For example, upon answering the telephone call, the answering modem will not provide the industry standard answering tone but, instead, provide a special sequence of characters. If the originating modem recognizes the special sequence of characters it will signal to the answering modem that it is of the same type, and both modems can then switch to a proprietary form of operation. However, if the originating modem does not recognize the special sequence of characters it will "time-out" and either assume a default operation or drop the line assuming that the no modem has answered.
As can be seen from the above prior art, there are basically two ways to provide identification between modems. One method first establishes the physical data connection before the originating modem and the answering modem attempt to identify each other through an additional handshaking procedure. Unfortunately, this only adds to the delay that already exists in establishing the physical data connection--a delay which is already on the order of 3 to 8 seconds depending on network delays. The second method uses a proprietary handshaking process before establishing the physical data connection, with the result that an incompatible originating modem may become confused and drop the connection. Consequently, it may be necessary to avoid any attempt at identification between the modems in order to eliminate this possibility--with the result that any non-standard industry features have to be manually administered when both the originating modem type and answering modem type are known a priori.